Electroplating process



Patented Nov. 6, 1934 ELECTROPLATING PROCESS William M. Phillips,

Birmingham, and Guy M. Cole, Detroit, Mich., assignors to General MotorsCorporation, Detroit, Mich., a corporation of Delaware No Drawing.Application April 3, 1931,

Serial No. 5275618 '1 Claims.

The invention relates to the electrodeposition of tin particularly uponsurfaces of ferrous metal, i. e., metal consisting largely or chiefly ofiron. More specifically, it is designed to provide a method whereby thecoating metal may be deposited and the surface of the base metal may becleaned or pickled, both in a single operation. Tin possesses markedadvantages as a coating for ferrous metalsurfaces. It provides a smoothbright surface of pleasing appearance and of considerable protectivevalue in resisting oxidation and corrosion of the base metal. Morerecently also it has been found that a thin deposit of tin on engagingsurfaces of parts subjected to friction, such as pistons and gears,imparts extraordinary antifriction properties to the surfaces. Theelectrodeposition of tin is therefore assuming increased industrialimportance.

It is necessary, of course, to clean the surface by removal of scale andother matter, such as oil and grease, before the plating operation andwe have discovered that it is possible to perform both the cleaningoperation and the plating operation in a single bath. by suitableregulation of the bath ingredients and proper coordination of thetreatments.

It is known that removal of scale and other materials adhering toferrous metal can be accomplished by treatment with a pickling acid,which may be, for example, sulphuric, hydrochloric, phosphoric, etc. Thecleaning operation is also facilitated by electrolytic action by passingan electric current through the bath and thus evolving hydrogen at thesurface of the metal to be cleaned. Since the electrodeposition of tinrequires a direct current through the bath to the article as cathode, weutilize the same current to facilitate the pickling operation and todeposit tin on the cleaned surface.

Tin may be deposited from acid solutions of its salts, such as thesulphates, chlorides, phosphates, etc., and we have found that thepickling acids, for example, sulphuric acid, may be added to the bath inconsiderable excess without interfering with the deposition of a smooth,dense and adherent coating of tin.

This fact makes it possible to increase the acid content to such extentthat it will remove scale and, atthe same time, by passing directcurrent through the bath, deposit tin. In order to secure the advantagesof electrolytic pickling, the current is supplied at such strength as tocause copious evolution of hydrogen at the cathode. The deposition issufficiently selective in action as the tin does not deposit on thescale but is deposited only on the unoxidized or cleaned surfaces. Sincethe plated surface is also resistant to corrosion by the pickling acid,the deposition of the tin also serves to arrest further attack by theacid upon the cleaned surfaces.

The proportions of tin salt and of acid may be varied over quite a widerange in accordance with the required conditions of cleaning andplating. The same is true of the amount of current required and theoptimum temperature range. Since tin is a relatively expensive metal, itis desirable, for economic reasons, to maintain as low a tinconcentration as is compatible with the production of a satisfactorydeposit. The time required for the cleaning operation is also animportant factor. The throwing power of the tin bath varies, roughly,directly with the concentration of the tin salt and inversely with thestrength of the acid. It will be understood therefore that the solutionmust be so constituted, with reference to the condition and character ofthe article, as to effect the desired cleaning operation in thepredetermined time and with the selected conditions of temperature andcurrent. These factors having been determined, it then remains to soadjust the contact of tin salt as to enable the article to be completelycovered in the time allowed, the solution being made more concentratedfor shorter periods of treatment since deposition takes place morerapidly from more concentrated baths. Solutions for commercial use havebeen prepared, using, for example, tin sulphate and sulphuric acid, withthe tin salt concentration varying from 1 gram per liter to 100 gramsper liter and the acid concentration varying from 5 to 20% (by volume).With these bath concentrations, the current was varied from 25450amperes per sq. ft. of cathode surface and the temperature from 65 F. to212 F. The effect of increasing the current and raising the temperatureis to increase the rate of cleaning with a given acid concentration andincrease of current will, of course, increase the rate of deposition.The tin salt preferably used is determined chiefly by economicconsiderations. The sulphate is cheap and is entirely satisfactory formost purposes. Tin may also be supplied in the form of various othersalts, for example, sodium stannate. Sulphuric acid is generallypreferred as the pickling acid because of its availability and other,advantages. With the sulphuric acid bath, anodes of pure lead orantimonial lead may be used as such anodes are sufliciently inert in thesulphate bath. Tin anodes. might be emp y d. as a source of metal, butthe use of tin salts renders it easier to control the bathconcentration. If hydrochloric acid is used as apickling acid it isdesirable to use anodes relatively inert in that acid, for example,carbon anodes.

There are two classes, chemically, of the tin salts, viz., the stannousand stannic salts. The stannous salts, being of lower valence, depositat greater efliciency than the stannic salts. However, the production ofsmooth, bright, adherent deposits of tin from solutions of stannoussalts, requires generally the presence of an addition agent, frequentlyof colloidal nature, such as glue (or related substances, as gelatin,dextrin, etc.), phenol, aloin, sulphonated castor oil, etc. Without theuse of one of these agents, which we term for convenienceadhesion-promoting addition agents, nothing more than spongy and loosedeposits are ordinarily obtained from stannous salts.

The stannic salts show much less tendency to form spongy or loosedeposits and do not generally require the addition agent. The rate ofdeposition and throwing power of such baths, however, are inferior tothose of the stannous salts, and, although some of the well-knownoxidizing agents may be employed to convert the stannous to stannicsalts and thus reduce the tendency to form loose deposits, the use ofthe stannous salts with an adhesion-promoting addition agent isconsidered preferable.

As a specific example of successful commercial operations, we havetreated forged heat-treated gears having on their surfaces a heavy scaleresulting from heat treatment. These gears were subjected to picklingand plating treatment in baths containing between 10 and 15% ofsulphuric acid (by volume) and about 1 /2 to 2 /2 grams per liter oftin. The latter was added in the form of some convenient salt of tin,for example, sodium stannate, the tin of which appears in the bath inthe stannic form, or stannous chloride, in which latter case one of theadhesionpromoting addition agents is employed e. g. about a half gram ofglue per liter. The current strength supplied was about 70 amps. per sq.ft. of cathode area and the temperature of the bath was about 150 F. Thetreatment was continued for from 3 to '7 minutes depending upon thecharacter of the scale. The gears come from the bath with scale entirelyremoved and with a smooth bright deposit of tin over the entire surface.

The use of our method enables a considerable saving to be made in time,equipment and expense over those involved in the usual tin platingmethods in which separate baths are required for cleaning and plating.Furthermore, the usual methods generally require one or more additionaloperations to protect the pickled surfaces and/or to clean them againpreliminary to the plating operation. The coating obtained is dense,smooth and strongly adherent and may be obtained in any desiredthickness by suitable adjustment of the tin content and current densitywith relation to the other factors. For economic reasons, however, athickness of not over one half a thousandth of an inch is recommended asthicker deposits can be more cheaply produced in separate plating baths.Even when thicker deposits are required, however, it is desirable insome cases to first treat the articles by the method herein set forthand increase the thickness of the deposit by a subsequent platingoperation in a more concentrated bath. Other protective metal coatingsadherent to tin, such as copper, may also be deposited directly on thetin coating.

We claim: v

1. The process of electrolytically removing scale from ferrous metalsurfaces and simultaneously electrodepositing tin thereon comprisingsubjecting said surface to the action of a bath containing ascale-removing concentration of a pickling acid while passing currentthrough the bath to the surface as cathode, to thereby evolve hydrogenat said surface, the bath also containing such concentration of tin saltthat the current employed will deposit a continuous adherent coating oftin on said surface in substantially the time required for the removalof scale therefrom.

2. The process as set forth in claim 1, said tin salt being a stannoussalt and the bath containing an addition agent adapted to promoteadhesion of tin deposited from such salt.

3. The process as set forth in claim 1, the bath containing sulphuricacid to the amount of approximately 5 to 20% by volume.

4. The process set forth in claim 1, the bath containing between 5 and20% of sulphuric acid, by volume, and a salt containing tin in amountsufficient to provide 1 to 2 grams of tin per liter.

5. The process of cleaning and coating an article having ferrous metalsurfaces comprising immersing the article in'a bath containing apickling acid in such concentration as to remove scale and also a tinsalt from which tin may be electrolytically deposited, supplyingelectric current to the article as cathode in the bath in quantitiesbetween 25 and 150 amperes per square foot of cathode surface, the acidconcentration being maintained such as to effect electrolytic picklingwith the current employed and the concentration of tin being maintainedsuch as to effect deposits of tin upon the cleaned surfaces sufiicientto protect the cleaned surface from further attack by the acid.

6. Process as set forth in claim 5, the content of acid being 5 to 20%by volume of sulphuric acid and the tin being present as a stannous saltin quantity sufficient to provide 1 to 2 /2 grams of tin per liter ofbath.

7. The process of treating articles of ferrous metal to remove scalecomprising placing the article in a bath containing a sufficientconcentration of pickling acid to effect scale removal, passing throughthe bath to the article as cathode an electric current of such strengthas to cause copious evolution of hydrogen at the cathode to assist inremoval of scale, maintaining in the bath a concentration of tin saltsof such character that tin can be deposited therefrom by the saidcurrent, and so adjusting the concentration of tin salts, the acidity,current density and temperature that a thin deposit of tin forms uponthe scale-free surfaces of the article in substantially the timerequired to remove the scale and of substantially only the thicknessrequired to protect the cleaned surface from attack by the acid.

WILLIAM M. PHILLIPS. GUY M. COLE.

